Rotary tables are being increasingly used to mount workpieces on coordinate-measuring instruments, since certain measurement tasks on the workpieces can be carried out more easily with the aid of a rotational axis as the fourth axis of measurement. For such a use it is necessary, however, that the angle values given by the rotary table and deviations of the rotary-table axis from an ideal axis of rotation be as small as posssible and that they do not increase the measurement uncertainty of the coordinate-measuring instrument. A summary of the problems inherent in such use of rotary tables is given in the paper, "Accuracy Specifications of Rotary Tables and Particulars on Their Use on Coordinate-Measuring Machines", by H. J. Neumann in VDI-Berichte 529 (1984).
The deviations of a rotary table can be broken down into the following components:
1. deviations of angular position PA1 2. deviations in travel of the rotary axis
(a) axial deviation PA2 (b) radial deviation PA2 (c) wobble deviation
These four components are regularly measured in the course of the acceptance testing of a rotary-table unit.
It is also already known to store the measured systematic angular position deviations of a rotary-table unit and take them into account as correction values in the calculation of the measurement results. If rotary table deviations are to be used for computational correction of measurement values, then not only the maximum value of each error component but the variation thereof as a function of the angle of rotation must, however, be measured. For this purpose, very different measuring methods have thus far been used.
To measure deviation in angular position, calibrated angle standards, preferably in the form of polygonal mirrors, are placed on the rotary table, the mirror surfaces of said mirrors with respect to their angular position being precisely known. Measurement of the angular position of the rotary table is then effected by observation of the mirror surfaces, using an auto-collimating telescope.
To determine axial deviation in travel of the axis of rotation, a ball is fixedly mounted at the center of the rotary table, or a flat disk is mounted perpendicular to the axis of rotation of the rotary table and is scanned continuously or at different angular positions by an inductive scanner that is positioned in the axis of rotation.
Radial deviation in travel is determined by scanning a centrally clamped cylinder or ring with a radially oriented path recorder, i.e., in an arrangement known in principle from roundness-test devices.
Finally, for the measurement of wobble deviation, a flat mirror is mounted on the rotary table with its surface perpendicular to the axis or rotation and is measured with an auto-collimating telescope; alternatively, two path recorders are oriented as for measurement of radial travel deviation, the recorders being axially offset from each other and being used in a difference circuit.
Until now, the acceptance testing of rotary-table units has been a tedious process, requiring a large amount of time and personnel, since a different measurement structure is used for each of the indicated components. As a consequence, rotary tables which have thus far been used have not been recalibrated at the situs of use of the coordinate-measuring instrument, nor have the calibration values been adapted as stored values for computational correction for the actual travel and position deviations. Such recalibration is, however, necessary under certain conditions since, in particular, systematic travel deviations of the axis of rotation are not completely stable over a long time but are subject to changes, due to running-in effects.
From Federal Republic of Germany OS 2,940,633 it is known, in order to determine the position of the axis of rotation of a rotary table on a coordinate-measuring instrument, to mount a measurement point on the rotary table and to determine the position of the measurement point for each of three angular table positions about the axis of rotation. By this method, however, only the average position of the axis of rotation can be determined but not its travel or position deviation.